Summary of Work: The aim of this six month inhalation study is to determine if exposure of transgenic mice to chloroprene will cause the same spectrum of tumors and target sites found in the 2 year NTP study using B6C3F1 mice. The results of this study will be used to investigate the mechanism of chloroprene carcinogenicity and to help validate the use of the transgenic model in carcinogenesis studies. The p53 deficient mouse was used because it appears to be an appropriate model for investigation of chemically-induced malignancies. The tumor suppressor gene p53 is frequently mutated or lost in tumors of mesenchymal origin. The lliza(lacIq) Big Blue mouse was used because it is an appropriate model for in vivo mutagenesis studies. The TG.AC strain was used as a model to evaluate the ability of chloroprene to induce skin and lung tumors. In addition, the FVB/N background strain was included as a control. All animals had the fur clipped on a designated area on the back to facilitate skin exposure to chloroprene. Transgenic mice were exposed by inhalation to 0, 2.0, 12.8, or 80 ppm chloroprene vapors 6 hours/day, 5 days/week for 6 months. Mice were evaluated grossly for first appearance of skin papillomas, palpable mammary tumors and other signs of toxicity. No chemical related tumors were detected upon gross examination. Skin papillomas were observed in control and treated TG.AC mice and a number of solid tissue tumors associated with the transponder implants were observed in p53 deficient mice; however, the incidence of these lesions was similar in control and treated groups. All mice were necropsied after 6 months of exposure and tissues were submitted for histopathological evaluation.